jmc-xavier

The effect of ply thickness on the onset of intralaminar and interlaminar damage is extremely important for the structural response of laminated composite structures. This subject has gained particular interest in recent years due to the introduction in the market of spread-tow, ultra-thin carbon-fibre reinforcements with different configurations. In the present paper, an experimental test campaign was carried out to study the structural response of aerospace-grade plain weave spread-tow fabrics (STFs) of different areal weights. The results showed that, in spite of an apparent superior longitudinal tensile strength of the thick STF, the multidirectional thin-STF laminate exhibited an improved tensile unnotched strength over the thick-STF laminate, attributed to its damage suppression capability. However, damage suppression was also responsible for similar tensile notched strengths. In compression, the thin-STF laminate performed substantially better than the thick-STF laminate in both unnotched and notched configurations. Finally, a similar bearing response was obtained in both STF laminates, in spite of a slightly higher resistance of the thin-STF laminate to the propagation of subcritical damage mechanisms.

Fracture characterization of human cortical bone under mode II loading was analyzed using a miniaturized version of the end-notched flexure test. A data reduction scheme based on crack equivalent concept was employed to overcome uncertainties on crack length monitoring during the test. The crack tip shear displacement was experimentally measured using digital image correlation technique to determine the cohesive law that mimics bone fracture behavior under mode II loading. The developed procedure was validated by finite element analysis using cohesive zone modeling considering a trapezoidal with bilinear softening relationship. Experimental load-displacement curves, resistance curves and crack tip shear displacement versus applied displacement were used to validate the numerical procedure. The excellent agreement observed between the numerical and experimental results reveals the appropriateness of the proposed test and procedure to characterize human cortical bone fracture under mode II loading. The proposed methodology can be viewed as a novel valuable tool to be used in parametric and methodical clinical studies regarding features (e.g., age, diseases, drugs) influencing bone shear fracture under mode II loading.

Abstract This work presents an experimental method to measure the compressive crack resistance curve of fiber-reinforced polymer composites when subjected to dynamic loading. The data reduction couples the concepts of energy release rate, size effect law and R-curve. Double-edge notched specimens of four different sizes are used. Both split-Hopkinson pressure bar and quasi-static reference tests are performed. The full crack resistance curves at both investigated strain rate regimes are obtained on the basis of quasi-static fracture analysis theory. The results show that the steady state fracture toughness of the fiber compressive failure mode of the unidirectional carbon-epoxy composite material IM7-8552 is 165.6kJ/m2 and 101.6kJ/m2 under dynamic and quasi-static loading, respectively. Therefore the intralaminar fracture toughness in compression is found to increase with increasing strain rate.

Abstract This paper reports the first attempt of developing macro-scale auxetic structures based on re-entrant hexagon design from braided composite materials for civil engineering applications. Braided composite rods (BCRs) were produced and arranged as longitudinal and horizontal elements to produce three types of auxetic structures: (1) basic re-entrant hexagon structure, (2) basic structure modified by adding straight longitudinal elements and (3): structure-2 modified by changing structural angle. The influence of various material and structural parameters as well as structure type on Poisson's ratio and tensile properties was thoroughly investigated. The auxetic behaviour was found to strongly depend on the structural angle and straight elements, resulting in lower auxeticity with lower angles and in presence of straight elements. Material parameters influenced the auxetic behaviour to a lesser extent and a decrease in auxetic behaviour was noticed with increase in core fibre linear density and using stiffer fibres such as carbon. The reverse effect was observed in case of tensile strength and work of rupture. Among these structures, structure-3 exhibited good auxetic behaviour, balanced tensile properties, and high energy absorption capacity and their auxetic behaviour could be well predicted with the developed analytical model. Therefore, these novel structures present good potential for strengthening of civil structures.

Abstract With the aim of characterising a commercially available epoxy adhesive used for fibre-reinforced polymers strengthening applications, when submitted to different environmental conditions, mainly thermal (TC), freeze–thaw (FT), and wet–dry (WD) cycles and immersion in pure (PW) and water with chlorides (CW) for periods of exposure that lasted up to 16 months, an experimental program was carried out. Several methodologies were used in its characterization, mainly the scanning electron microscope (SEM), dynamic mechanical analysis (DMA), standard tensile tests (STT) coupled with digital image correlation (DIC). In general the results revealed that the chemical composition was not affected by the environmental conditions. Nevertheless, it was verified through \{DMA\} and \{STT\} that the modulus of elasticity and tensile strength of the epoxy adhesive increased in the TC, while the specimens submitted to \{PW\} and \{CW\} faced a high degradation in terms of its mechanical properties. Eventually, the glass transition temperature (Tg) was not affected by the environmental conditions, apart from the specimens subjected to \{TC\} and FT, presenting a higher and lower Tg, respectively, when compared with the reference specimens.

Abstract Mixed-mode I+II fracture characterization of human cortical bone was analyzed in this work. A miniaturized version of the Single Leg Bending test (SLB) was used owing to its simplicity. A power law criterion was verified to accurately describe the material fracture envelop under mixed-mode I+II loading. The crack tip opening displacements measured by digital image correlation were used in a direct method to determine the cohesive law mimicking fracture behavior of cortical bone. Cohesive zone modeling was used for the sake of validation. Several fracture quantities were compared with the experimental results and the good agreement observed proves the appropriateness of the proposed procedure for fracture characterization of human bone under mixed-mode I+II loading.

The robustness of the test method based on a single 3D off-axis prismatic specimen for the simultaneous identification of the orthotropic stiffness components of clear wood is addressed. In this method, the specimen is consecutively submitted to uniaxial compression tests along its three orthogonal axes. A data reduction based on anisotropic elasticity is applied to extract active material parameters from 3D full-field deformation measurements provided by stereo-correlation over adjacent faces. Two major limitations of this test method, directly affecting the parameter identification, are analysed and discussed: (1) off-axes angle orientation; (2) friction effects. A numerical study pointed out that radial and tangential rotations of about 29° and 9°, respectively, balances out the strain components in the specimen response. Moreover, friction can be reduced by using mass lubricant or soft material in the contact interface, realising transverse shear deformation.

Abstract This work presents an experimental study on the effect of ply-level hybridisation on the tensile unnotched and notched response of composite laminates. In a first assessment, notched tests were performed on laminates with nominal ply thicknesses between 0.03 mm and 0.30 mm. From the understanding of the effect of ply thickness on the damage mechanisms that govern the notched response of laminates, the concept of ply-level hybridisation is introduced, which consists in combining plies of different grades. A uniform combination of thin and conventional plies resulted in a hybrid laminate with intermediate notched response. Selective hybridisation, where thin off-axis plies are combined with thicker 0° plies, resulted in a globally enhanced notched behaviour without compromising the unnotched and fatigue responses. This work clearly shows how ply-level hybridisation, when designed to trigger specific damage mechanisms, can be used to improve the notched response of composite laminates.

A new model based on Finite Fracture Mechanics (FFMs) has been proposed to predict the open-hole tensile strength of composite laminates [1]. Failure is predicted when bothstress-based and energy-based criteria are satisfied. This model is based on an analytical solution, and no empirical adjusting parameters are required, but only two material properties: the unnotched strength and the fracture toughness. In the present work, an extension of the proposed FFMs model to predict the notched response of composite laminates with notch geometries other than a circular opening [2] is presented and applied to the prediction of size effects on the tensile and compressive notched strength of composite laminates. The present model is also used to assess the notch sensitivity and brittleness of composite laminates by means of versatile design charts and by the identification of a dimensionless parameter designated as notch sensitivity factor. A further extension of the FFMs model is proposed, which takes into account the crack resistance curve of the laminate in the model's formulation, and it is used to predict the large damage capability of a non-crimp fabric thin-ply laminate [3].

Abstract The near-surface mounted (NSM) strengthening technique is capable of effectively increase the bearing capacity of structural concrete elements. This technique which basically consists of placing \{FRP\} reinforcements inside small grooves cut in the concrete cover, has been widely investigated in terms of structural performance and ability to improve the flexural and shear behaviour of reinforced concrete beams and columns. However, little research has been carried out concerning to the \{NSM\} long-term performance and durability. Motivated by the need of increasing the knowledge on the expected durability of the \{NSM\} technique using \{CFRP\} laminates, this paper presents an experimental program in which direct pull-out tests are carried out for evaluating the bond behaviour of specimens aged through wet-dry cycles. A total of 30 specimens are tested, analysing the effect of the bond length, the groove width, the groove depth and the aging effect on the bond behaviour. Digital image correlation method is also used to identify the bond resistant mechanism developed in an element strengthened using \{NSM\} technique. Finally, using the experimental results, an analytical�numerical strategy is applied to establish the local bond stress�slip relationship.

Abstract The bond behavior between Fiber Reinforced Polymers (FRPs) and masonry substrates has been the subject of many studies during the last years. Recent accelerated aging tests have shown that bond degradation and \{FRP\} delamination are likely to occur in FRP-strengthened masonry components under hygrothermal conditions. While an investigation on the possible methods to improve the durability of these systems is necessary, the applicability of different bond repair methods should also be studied. This paper aims at investigating the debonding mechanisms after repairing delaminated FRP-strengthened masonry components. FRP-strengthened brick specimens, after being delaminated, are repaired with two different adhesives: a conventional epoxy resin and a highly flexible polymer. The latter is used as an innovative adhesive in structural applications. The bond behavior in the repaired specimens is investigated by performing single-lap shear bond tests. Digital image correlation (DIC) is used for deeper investigation of the surface deformation and strains development. The effectiveness of the repair methods is discussed and compared with the strengthened specimens.

Abstract Direct identification of cohesive laws in modes I and İI\} of wood bonded joints is addressed by the double cantilever beam (DCB) and end-notched flexure (ENF) tests, respectively. Moreover, the development and extension of fracture process zone (FPZ) ahead of the initial crack tip, is analysed by means of digital image correlation (DIC) and embedded fibre Bragg grating (FBG) sensors. From \{FBG\} spectral response, the spectrum geometric mean is determined and the strain induced by wavelength variation employed to identify the initial and final stages of the FPZ. These stages are used to consistently define the cohesive laws in both modes I and II. Resistance-curves are determined from the compliance-based beam method (CBBM). Besides, the crack tip opening displacements (CTOD) are determined by post-processing displacement field provided by \{DIC\} around the initial crack tip. The strain energy release rate as a function of the \{CTOD\} are then determined for both mode I and mode II. The respective cohesive laws are reconstructed by numerical approximation and differentiation. It is concluded that the proposed data reduction scheme is effective to determine both the \{FPZ\} development phase and the corresponding cohesive laws of wood bonded joints in both mode I and mode II.

Abstract The strain rate dependent mechanical behaviour was studied for the common out-of-autoclave aerospace textile composite 5-harness-satin carbon�epoxy. End-loaded 15 � , 30 � and 45 � off-axis and 90 � compression tests were carried out at three different strain rate levels ( 4 � 10 - 4 s - 1 , 200 s - 1 and 1000 s - 1 ) to determine the effect of strain rate for transverse compression and combined transverse compression/in-plane shear loading. The dynamic tests were carried out on a split-Hopkinson pressure bar, where high speed photography and digital image correlation allowed a detailed study of the specimen deformation and failure process. Quasi-static reference tests were carried out on an electro-mechanical test machine using the same specimen type and a static \{DIC\} system. Pronounced strain rate effects on the axial stress�strain response were observed for all specimen types. Failure envelopes for the combined s 22 c - t 12 stress state were derived from the experimental data and compared with the maximum stress criterion, which appears well suited to approximate the experimental failure envelope at all strain rate levels. It was observed that the failure envelope was simply scaled up with increasing strain rate, while the overall shape was found to be strain rate independent.

Abstract A modified Iosipescu specimen is proposed to measure the mode İI\} intralaminar fracture toughness and the corresponding crack resistance curve of fibre reinforced composites. Due to the impossibility of scaling the specimen, a modification of the classical size effect method is proposed. The calculation of the crack driving force curves is performed using the Finite Element Method. The classical Iosipescu shear feature was used and tests were coupled with digital image correlation to support the proposed approach. Experiments were performed on IM7/8552 material system and the R-curve was obtained. The steady-state value of the fracture toughness of the ply is found to be equal to R 0 ss = 34.4  kJ/m2.

Abstract This work addresses the experimental identification of mode I cohesive law of wood bonded joints. The approach combines the double cantilever beam (DCB) test with both digital image correlation (DIC) and embedded fibre Bragg grating (FBG) sensors. The spectrum geometric mean of the \{FBG\} reflected spectral response was determined, and the wavelength evolution was used to define the fracture process zone (FPZ) development phase. This evaluation allowed a consistent selection of experimental range of over which the identification procedure of mode I cohesive law is build up. Mode I crack length, Resistance-curve and cohesive law parameters are characterised and discussed. The strain energy release rate (GI) is determined from the P�d curve by the compliance-based beam method (CBBM). The crack tip opening displacement (wI) is determined by post-processing displacements measured by DIC. The cohesive law in mode I (sI�wI) is then obtained by numerical differentiation of the GI�wI relationship.

Abstract Biodegradable polymers such as poly(lactic) acid (PLA) have been studied for biomaterials applications such as natural human ligament replacement, however these materials could be applied to other sectors as aerospace, aeronautics, automotive, food packaging. \{PLA\} presents a relatively brittle with a mode I fracture behavior, being often blend with other biodegradable or non-degradable polymers to improve its fracture energy. For some existing applications, \{PLA\} components exhibit accumulated permanent deformation resulting from dynamic mechanical inputs, resulting on failure by laxity of parts. Aiming the improvement of \{PLA\} mechanical properties, the inclusion of carbon nanofillers into \{PLA\} matrix, in particular, CNT-COOH and \{GNP\} have been developed, due to their strong sp2 carbon-carbon bondings and their geometric arrangement that enhance mechanical properties of the polymer matrix. \{PLA\} and nanocomposites were produced by melt blending followed by compression molding in a hot press, with small weight percentages of nanofillers added to the matrix. Quasi static tensile tests were performed on a mechanical testing machine (Instron™ ElectroPuls E1000) along with failure analysis of specimens with centered crack with digital image correlation, revealing strain distribution along specimens.

The direct identification of the cohesive law in pure mode I of Pinus pinaster is addressed. The approach couples the double cantilever beam (DCB) test with digital image correlation (DIC). Wooden beam specimens loaded in the radial-longitudinal (RL) fracture propagation system are used. The strain energy release rate in mode I (GI ) is uniquely determined from the load-displacement ( P ?? ) curve by means of the compliance-based beam method (CBBM). This method relies on the concept of equivalent elastic crack length ( eq a ) and therefore does not require the monitoring of crack propagation during test. The crack tip opening displacement in mode I ? ? I w is determined from the displacement field at the initial crack tip. The cohesive law in mode I I I (? ? w ) is then identified by numerical differentiation of the I I G ? w relationship. Moreover, the proposed procedure is validated by finite element analyses including cohesive zone modelling. It is concluded that the proposed data reduction scheme is adequate for assessing the cohesive law in pure mode I of P. pinaster.

A miniaturized version of the double cantilever beam (DCB) test is used to determine the fracture energy in human cortical bone under pure mode I loading. An equivalent crack length based data-reduction scheme is used with remarkable advantages relative to classical methods. Digital image correlation (DIC) technique is employed to determine crack opening displacement at the crack tip being correlated with the evolution of fracture energy. A method is presented to obtain the cohesive law (trapezoidal bilinear softening) mimicking the mechanical behavior observed in bone. Cohesive zone modeling (CZM) (finite-element method) was performed to validate the procedure showing excellent agreement.

Abstract The strain rate dependent mechanical behaviour was studied for the common out-of-autoclave aerospace textile composite 5-harness-satin carbon–epoxy. End-loaded 15 ° , 30 ° and 45 ° off-axis and 90 ° compression tests were carried out at three different strain rate levels ( 4 × 10 - 4 s - 1 , 200 s - 1 and 1000 s - 1 ) to determine the effect of strain rate for transverse compression and combined transverse compression/in-plane shear loading. The dynamic tests were carried out on a split-Hopkinson pressure bar, where high speed photography and digital image correlation allowed a detailed study of the specimen deformation and failure process. Quasi-static reference tests were carried out on an electro-mechanical test machine using the same specimen type and a static \{DIC\} system. Pronounced strain rate effects on the axial stress–strain response were observed for all specimen types. Failure envelopes for the combined σ 22 c - τ 12 stress state were derived from the experimental data and compared with the maximum stress criterion, which appears well suited to approximate the experimental failure envelope at all strain rate levels. It was observed that the failure envelope was simply scaled up with increasing strain rate, while the overall shape was found to be strain rate independent.

Fracture characterization under mode I loading of a cement-based material using the single-edge-notched beam loaded in tree-point-bending was performed. A new method based on beam theory and crack equivalent concept is proposed to evaluate the Resistance-curve, which is essential to determine fracture toughness with accuracy. The method considers the existence of a stress relief region in the vicinity of the crack, dispensing crack length monitoring during experiments. A numerical validation was performed by finite element analysis considering a bilinear cohesive damage model. Experimental tests were performed in order to validate the numerical procedure. Digital image correlation technique was used to measure the specimen displacement with accuracy and without interference. Excellent agreement between numerical and experimental load–displacement curves was obtained, which validates the procedure.

Abstract Probabilistic fatigue models are required to account conveniently for several sources of uncertainty arising in the prediction procedures for structural details, such as the scatter in material behaviour. In this paper, a procedure to derive probabilistic S-N fields for structural details [1] is applied to a notched geometry with elliptic central hole made of puddle iron from the Eiffel bridge, based on the local approaches supported by the probabilistic ɛa–N or Smith–Watson–Topper (SWT)–N fields [1,2]. This procedure suggests an extension of the fatigue crack propagation model proposed by Noroozi et al. [3,4] to structural details, in order to cover both the fatigue crack initiation and fatigue crack propagation, based on local strain approaches to fatigue. Both fatigue crack initiation and fatigue crack propagation mechanisms are accounted for in the proposed approach. The numerical results are compared with available experimental S-N fatigue data for the notched plate under consideration.

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